Dave (and others)
From my pov, all summed above. It can have it's place, especially when seen in the wider entire test phase e.g. at some point an IAS / Alt comparison can be made - although since the chase will often be (need to be!) an RV, you need to ask how well calibrated that one is
Our "regulator" over here (the LAA) gives a list of the "aims" / "targets" of the 1st flight, and Formation is not one of them. It will only be a 15-20min flight to achieve those aims, and will be high power early on, and then some slow speed stuff, so hard for anyone else to formate on, especially since the target aircraft will not be over-helpful.
Years ago I went to support a friend on his first flight of an RV 4. Half the RV community in a large Metro-area was there too. He must have had 5+ chase planes
Something I was told, and agree with, is keep those who "know" of the 1st flight, as small as possible. This reduces pressure to make the flight with either aircraft or weather issues. We can just walk away and await the issues solved without leaving anybody else any the wiser.
Clearly if you the resources, experience and plan to do it "properly" (and above all discipline) then fine e.g. as with Paul. But I don't think anyone should feel pressurised to try and emulate that without the other elements in place...